Oil recovery process



nited States Patent OIL RECOVERY PROCESS Harry L. Pelzer, Catoosa,0kla., assignor to Sinclair Oil & Gas Company, Tulsa, Okla., acorporation of Marne No Drawing. Application March 5, 1954, Serial No.414,489

4 Claims. (Cl. 166-11) My invention relates to a process for therecovery of oil and gas from oil-bearing underground reservoirs bythermal means. More particularly, it relates to a step- .wise processwherein initially a heat wave is propagated within the formation andfinally the heat content of the wave is employed to finish the recoveryprocess by a hot water drive.

In order to recover oil from an underground reservoir by means of wells,energy is required to bring the oil into the well bore from remoteportions of the formation. It is well known however that the energycontent of the original undisturbed reservoir, as a generality, isinsufficient to recover all of the oil. Even in the unusual case ofreservoirs containing sufiicient gas under pressure to provide thetheoretical energy requirements for complete oil removal, a largeproportion of the energy is wasted because the gas escapes from theproducing wells Without bringing oil to the well bore in amountscorresponding to the available energy. Hence primary pro duction rarelyif ever results in complete recovery of the oil. Although injection ofextraneous water or gas can be used to supplement the natural energy ofa reservoir or, in secondary recovery methods, to supply energy inreservoirs which have been depleted in energy, it is common knowledgethat these methods, whether applied simultaneously or successively, donot recover all of the oil, probably because of viscosity, capillarityand adsorption effects within the formation. v

Thermal means have been often proposed for more efiicient oil recoverywhereby thermal energy is' introduced into the reservoir by means of hotliquids or gases or is generated within the formation by in situcombustion of part of the oil or of other combustibles. In thesemethods, the heat generated at one point, either in the injection wellbore or at the combustion point Within the formation, must be moved toother points in the formation. Usually, a gaseous heat carrying mediumis employed which transmits the thermal energy to other parts of theformation and at the same time functions as a source of mechanicalenergy as in the case of a gas drive. Improvement in oil recoveryefiiciency then .may

result by reason of the increased temperatureof the oil which decreasesits viscosity and by cracking and distillation of the oil, but majorimprovement in recovery is most effective if the temperature isincreased high enough, so that it can be moved toward the producing wellas a vapor rather than as a liquid.

The present invention is based on findings developed in the course ofextensive investigation of thermal recovery both in the laboratory andin the field. Thus I have found that the use of water to utilize theheat generated by an initial combustion drive provides means forrecovery of oil by a thermal process which eliminates disadvantages ofpreviously proposed thermal methods.

My process is based in part on the discovery that a heat wave propagatedin the formationby in situ combustion caused by injection of a leanmixture of oxygen-containing gas (fuel content less than lower explosivelimit) through an injection well toward a producing well results in abroadening annular band or wave of peak temperature. This is surprisingsince attenuation of the wave because of its ever increasing radius fromthe input well would be expected. My invention also applies thediscovery that in situ generation of steam results in substantiallycomplete removal of oil from the formation. Thus the oil is made mobileand recoverable at a temperature corresponding to the boiling point ofwater at the subsurface pressure (considering capillary effects, 200 F.to 1000 F.) prevailing so that temperatures in the cracking range can beavoided in the presence of steam. v r

In the practice of the invention, a high temperature heat wave isestablished within the formation. Advantageously, the heat wave isestablished by burning a fuelair mixture in the bore of an input well atformation level. The fuel-air mixture is proportioned to provide atemperature within the range of about 1000 to 2000 F. A suflicientperiod of preheating is provided to heat a substantial portion of therock formation surrounding the well bore to a temperature above theignition temperature for hydrocarbons in the formation, i. e. aboveabout 450 to 500 F. After the oil-bearing strata around the well borefor a radius of several feet has been heated to high temperature,preferably above 1000" F., combustionin the well bore is terminated. Theheatedzone isnow moved into the formation by injecting unheated,noncombustible gas such as air or air containing an amount of fuel gasbelow the explosive limit. Alternatively, air and fuel gas may beinjected alternately in a manner preventing burning within the wellbore. Alternatively fuel gas containing an oxygen content below the richexplosive limit may be used for movement of the heated zone into theformation. The gases enter the well bore cold and pick up preheat fromthe rock face. By transfer of heat outwardly through the rock the heatedzone is moved away from the well bore. Once the well bore has beencooled below the ignition point a heat wave in the sense of theinvention has been established which can be propagated within theformation. An oxygencontaining gas drive is employed to propagate theheat wave, advantageously air at 'as high an input rate as practicabletaking into account the permeability of the rock, power requirements forcompression and pressure limitations. The input rate and partialpressure of oxygen are maintained in any event high enough to insurecombustion of hydrocarbon residues in the rock as the oil is releasedand moved out into the formation before the advancing heat Wave. If theresidue is too lean to provide sutficient heat to maintain the peaktemperature of the advancing wave at a high level, e. g. about 1000 F.,fuel gas in a proportion below lean explosive limits may be injectedwith the injected air. The injected gas progresses radially out into theformation from the input well and is preheated to above the ignitiontemperature as it passes through the peak temperature zone. As thepreheated air comes into contact with residual fuel at the leading edgeof the wave, the in situ combustion process is continued, generatingadditional heat and thus propagating the heat wave in the form of anadvancing annular ring.

The total heat content of the wave is increased by providing a highpartial pressure of oxygen. It is most practical to use air, or a lowerconcentration of oxygen than contained in air. Under these conditions, Ihave found that the breadth of the wave continuously increases withv thegas to increase the heat carrying capacity of the injection medium, thusfurther accelerating the movement of heat through the improvement inheat transfer capacity, without loss of combustion. For example, amixture comprising 12 cubic feet of air or 12 cubic feet of a perfectlycombustible air-gas mixture per pound of Water may be employed. Theobject is to furnish the maximum sensible heat consistent withmaintenance of a peak temperature above the ignition point for residualhydrocarbons within the formation.

Although the entire recovery process can be conducted by thermal meansby proceeding in this manner until the heat wave reaches the producingwell or wells, it is an essential feature of my invention to switch fromthe in situ combustion step to in situ steam drive by replacing airinjection with water injection. The change is made once theheat contentstored in the rock is suificient to carry the heat wave without furthercombustion by means of water drive to the producing well or wells at atemperature equivalent to the boiling point of water under the pressureconditions prevailing at the producing Well or wells. Operating in thismanner, maximum utilization of heat is obtained without sacrifice inrecovery eiliciency. Combustion is continued only long enough to providefor release of the optimum quantity of heat and in situ steam generationhas been found to effect substantially complete release of recoverableoil from the formation. Since a much lower temperature is used, that isa temperature corresponding to the boiling point of Water under thepressure prevailing at the producing well, instead of the combustiontemperature, less cracking of the oil occurs. The result is improvementin liquid recovery and in quality of the liquid product.

The invention may be illustrated by way of an example of a fieldoperation designed to accomplish tertiary recovery of oil from theBartlesville formation in the Nowata- Delaware area of Oklahoma.Representative average data for the formation follow: Thickness 30',porosity 20%, average permeability 150 millidarcys, oil saturation 35%,water saturation 35%, gas saturation 30%, true density of rock 160pounds per cubic foot.

A five-spot pattern in which the number of input wells is equal to thenumber of producing wells is used although the description is confinedto one element of the five-spot pattern. The well spacing employed is330 feet between like Wells, or

from input well to producing well. In conducting the operation accordingto the example, the following additional basic data are applied or areassumed.

Temperatures: Initial formation temperature 70 F. (as found in theBartlesville sand at Nowata); peak temperature of the wave 1000 F. (frompetrographic examination of cores from field trials, corroborated bylaboratory tests); final temperature of steam drive 300 F.

Heat capacities: Gas 0.02. B. t. u./F./s. c. f.; rock 0.2 B. t. u/F/lb.;water 1500 B. t. u./lb. between 70 F. and 1000 F., and 1200 B. t. u./lb.between 70 F. and 580 F. (from standard Mollier diagram).

Residual saturation after steam drive: 6% oil, 85% water, 9% gas.

Heat generated inburning; 500 B. t. u./s. c. f. of oxygen, 20,000 B. t.u./lb. of oil.

Injection rates: 1,000,000 s. c. f./day of air (oxygen content: 20%),oxygen in the produced gas 2% (as determined by field trials). B./D.

The preliminary operations include installation of a burner of the typedescribed in application Serial No. 97,142, filed June 4, 1949 of JohnJ. Piros and Oliver P. Campbell at formation level. The burner isignited and hydrocarbon gas is burned with air in the bore of theinjection well in a manner injecting the resultant exhaust gases intothe formation. The well-burning operation Water injection rate: 200

recycle.

4 is continued until the oil bearing strata have been heated for aradius of several feet around the well bore to a temperature of theorder of 1500 F. Combustion in the well bore then is stopped byswitching to injection of unheated air or field gas.

In practicing my invention the high temperature front is advantageouslyestablished within the formation by burning fuel, conveniently naturalgas or crude oil, with air at high temperature either on the surface, orpreferably within the hole. The resulting combustion gases underelevated pressure are forced into the porous oilbearing stratum for alength of time suflicient to raise the temperature of a large body ofsand surrounding the well to a temperature below the fusion temperatureof the rock but Well above the ignition temperature of the residualcarbon. At the same time, the formation is partially re-pressured withresulting flow of cold oil and gas toward the outlet wells.Alternatively, however, other ignition means such as chemical ignitersand fuels may be employed, switching to air drive once ignition isaccomplished.

During the fuel-burning period, there is normally a large excess ofoxygen in the combustion gases because of the use of dilution air in theburner system, which insures clean combustion and prevents the formationof soot that might clog the formation, and additionally assists inheating up the oil bearing rock by burning carbonaceous residue and someoil.

The use of a bottom-hole burner is advantageous because it eliminatesthe need for expensive alloy casing and expansion joints. However, thecombustion gases may be produced by burning at the surface in anydesired type of combustion system producing a stream of hot gas underhigh pressure or a system combining a surface preheater and bottom-holeburner may be used. With bottom-hole combustion, various ignitionschemes may be used, for example electric sparking, thcrmite, or otherincendiary bombs. However, a jet-type, spark ignited combustion systemwith a high velocity, turbulent air and gas flow in which a relativelylarge volume of diluent air or gas is introduced progressively into theburning zone provides a convenient and reliable bottom-hole system.Recycle gas may be utilized as diluent after the temperature has beenbuilt up, and where gas engine driven compressors are employed, theengine exhaust may be utilized as recycle or diluent gas.

The inlet pressures Will vary according to the distance betweenproducing and input wells, the thickness and permeability of theoil-bearing stratum, and the oil and water content of the formation.Similarly, the quantity of gas introduced will be affected by thedesired pressure, temperature of the input gases, and the conditions andheat capacity of the oil-bearing stratum. The pressure for example willordinarily exceed 60 p. s. i. 55., but because of problems of reservoircontrol is maintained at a moderate figure and, of course, is ultimatelylimited by the overburden. To minimize plugging or cementing of theporosity of the formation, after-scrubbers or other filtering devicesshould be employed on compressors in order to remove iron rust or othertroublesome carry-over. Similarly, it is desirable to incorporatesimilar devices in the liquid knock-out system employed for handlingrecycle gas.

The gas produced is recovered and after recompression to compensate forpressure drop may be recycled. Main- Depending upon its fuel content,the produced gas may be burned as fuel or utilized as gas recycle.Readily liquefiable or other valuable components may be recovered as byabsorption prior to utilization as Pressure drop is desirably kept lowand may be controlled by keeping the volume gas rate low and byoperating at a high pressure level.

Analysis of the produced gas, as by the Orsat method,

was er.

for oxygen and carbon dioxide content provides ameans for determiningthe state and progress of the front within the formation. It is helpfulto observe pressure differentials between inlet and outlet flow, whichare afiected by the temperature, the permeability, whether fusion isoccurring, whether an oil and water block is building up, and theposition of the heat transfer point and the revivifying combustionpoint. Control timing is also assisted by the use of deep-wellthermometers or temperature recording devices.

In applying the invention to large scale recovery operation, it isadvantageous to utilize a logically spaced pattern of input and outletwells. In many of the oil fields which have been extensively gaspressured or water flooded, wells have been drilled in S-spot or 9-spotpatterns which will be suitable for applying the invention to furtherrecovery. It may be necessary, however, to drill a new input well or topull the old casing and replace it with pressure tight piping. The holesmay be tightly cemented with a high temperature resistant cement at thetop of the formation where necessary to confine the combustion andrecycle gases within the formation stratum. If non-uniform permeabilityis indicated by core analysis or erratic production, the use of wellpackers can be employed to seal off excessively permeable strata.

In establishing the wave, it is important to regulate the temperatureduring the period of combustion within the bore of the input well byregulation of the proportions of secondary air in the burner.Advantageously, temperature is maintained in the range of 1000 to 2000F., preferably about 1500 F. Too low a temperature will delaydevelopment of the heated zone because of poor ignition. Too high atemperature will cause sintering and spalling of the sand. Combustion inthe porous rock may be initiated with a gas-air mixture in thenon-explosive range, e. g. below 4% methane in air; after combustion hasbeen initiated, it is continued by charging in the non-explosive range,e. g. below 4% methane in air. By way of example, a mixture providing aheat release of 30 B. t. u. per cu. foot of air may be employed, but asthe temperature builds up to the ignition temperature of hydrocarbons inthe formation e. g. 500 F., the fuel content is dropped from the 30 B.t. 11. per cu. foot to 10 B. t. u. per cu. foot of air With propersecondary air control on the burner to give a temperature rise to withinthe l000 to 2000 F. range, about 5 to 8 days heating time should besufiicient to establish the heated area at the bottom of the injectionwell. It is necessary to stop combustion at the well bore so that bycooling the sand face the heated zone may be moved out into theformation. Thus, the wave form or profile is established as the trailingedge appears. During the period of cooling and movement of the hot zone,an unheated gas mixture, which is either non-combustible or which ishandled in a manner preventing combustion at the sand face or back, mustbe employed. For example, using air the fuel content should not exceed40 B. t. u. per cu. foot of air or a stationary heat front will develop.It is desirable to use air because the excess of oxygen expands the wavemore rapidly and contributes a greater amount of stored heat in the heatwave. As illustrated in the above example, the propagation of the waveis continued until sufficient heat is built up in the formation inrelation to the volume and porosity of the unheated portion of theformation between the leading face of the wave and the producing wellsto keep the water injected in the water drive above the boiling point upto the end of the recovery process, taking into account the pressure anddepth of the well. During the propagation step, air alone, air inadmixture with recycled field gas and/or fuel gas may be employed as thedriving medium. The use of fuel gas may be necessary to maintain thedesired peak temperature if the carbonaceous residue left in the sandupon release and movement forward of the bulk of the oil content isinsufficient for the purpose. In this case. the proportion of fuel mustbe regulated below or above the explosive level,

or it must be injected in alternate cycles with the air, to prevent burnback from the peak temperature zone and development of a stationaryfront rather than a moving wave. The use of field gas, recycled underpressure, reduces the compressor load for the gas injection operation.In using air, a small amount of fuel gas, say 1 to 5 B. t. u. per cubicfoot is advantageous as a tracer to indicate the position of thetrailing face of the heat wave. A window Well can be drilled and thepresence or absence of carbon dioxide in gas produced from it can bedetermined by analysis. Reasonably definite location of the leading andtrailing faces of the heat wave is important in determining the optimumpoint in the process for changing from the wave propagating drive to thewater drive. For with the width of the heat wave established, its heatcontent can be calculated by simple computation from available thermaland formation data as illustrated in the above example and then comparedwith the quantity of heat necessary to provide the desired terminationtemperature for the hot water drive, usually about 300 to 400 F., butequivalent to the boiling point of water at the pressure prevailing inthe producing well.

When straight air is utilized as the wave propagating medium, theleading face of the wave may travel as much as 3 times as fast as thetrailing face. Although the heat transfer wave corresponding to thetrailing portion and the combustion wave corresponding to the peaktemperature portion may be made to coincide by appropriate control oftotal average oxygen input as described in U. S. Patent No. 2,642,943,it is advantageous to introduce water with the wave propagating mediumto accomplish a similar purpose. Because of the much higher heatcapacity of water, pound for pound the horsepowerrequirements forinjecting the propagating medium may be markedly reduced by waterinjection. In the wave propagating step, therefore, injection of waterin an amount in the approximate range of 2 to 10 gallons per thousandcubic feet of gas is recommended. The water injected with the gas in thepropagating step also improves oil recovery by providing in situ'steamgeneration ahead of the peak temperature portion of the wave. Althoughthe water is vaporized as it passes through the wave, it is condensedsubsequently ahead of the wave and provides suffic-ient liquid watersaturation in the formation to promote desorption of the oil from thematrix as the wave approaches and revaporizes it. Thus only the heaviestresidual portion of the oil which would be unrecoverable in any eventremains behind to serve as fuel in the propagation of the heat wave.

By finishing the process with a water drive providing in situ steamgeneration to utilize the heat content of the wave, several substantialadvantages are obtained. The total quantity of heat required for acomplete recovery process may be reduced as much as a half. Similarly,the power requirements for injecting the drive media are tremendouslyreduced. The time required to attain complete recovery is less. Inaddition more certain recovery of the oil banked ahead of the heat waveis effected in view of the greater efiiciency of water as an oildisplacement medium than gas. Also oil consumption by combustion andcracking to gaseous hydrocarbons is avoided in the in situ steamgeneration phase of the operation.

The elfectiveness of oil removal and recovery has been established infield tests on a portion of the Bartlesville formation and has beenconfirmed by numerous laboratory experiments. Using a single input wellwith a ring of producing wells in a 34-foot bed of sand, 295 MM cubicfeet of air-fuel gas mixture was injected over a period of approximately10 /2 months. During the I test period, 34.6 MM cubic feet of oxygenwere consumed. By oxygen balance, the burned area was calculated to be acircle of about feet to feet in diameter. Two window wells were drilledinto the formation,

7 however, 50 feet and 170 feet respectively from the injection well,and by means of cores and temperature readings taken from the two windowwells the actual extent of the burned area was estimated at about 120feet.

After 10 months, the leading edge of the heat wave was at 58.7 feet fromthe injection well with the trailing face at. 41.5 feet as determined bya balance of input and output oxygen, assuming that each cubic foot ofoxygen burned liberated 500 B. t. u. and that the peak temperature ofthe sands reached 140 F. The position of the trailing face was confirmedby temperatures taken in the nearer Window well. Heat losses, based upondata calculated from temperatures taken at different levels in drillingthe window wells, were indicated as 2% maximum to overand under-burdenand possibly5% by conduction parallel to the wave. Further figures,based upon daily injection of 1.0 MM cubic feet of air for an additional300day period, showed that the front would be 104 feet from theinjection well with the rear face at 73.5 feet. The significance ofthese figures is that the width of the wave is ever widening so thatthere is a vast quantity of stored heat available for hot water drive atthe proper point in the operational sequence. The data also show thatthe possibility that combustion will be lost during the heat wavepropagation step is remote since preheat is always available forre-ignition.

Cores taken from the nearer window well showed that at least 93% of theoil in place had been removed. By balancing actual oil production fromthe ring of producing wells, oil moved out into the formation beyond thering of producing wells as obtained by averages of oil-gas ratios at theproducing wells, the liquid or residual oil converted to low 8. t. u.gas-air mixture and the liquid gas or residual hydrocarbons actuallyconsumed by oxygen and recovered or exited as carbon dioxide, data onhydrocarbon recovery were obtained. Test data available indicated that90% liquid oil recovery may be expected from the heating period followedby steam drive produced in situ by water injection, from oil sandscontaining 600 barrels of oil per acre foot, at the start of thecombined operation.

I claim:

1. Inthe recovery of oil from an oil-bearing underground formation bythermal means wherein a heat wave is established within the formation,the steps comprising heating the formation around the bore of an inputwell to establish a heated zone in the formation thereabout having atemperature above about 500 F., moving the said heated zone out into theformation and cooling the input well bore by injection of a coolingnon-combusting gas therein, and thereafter discontinuing the use of theinput well bore as a heat source, propagating the heat wave within theformation by injection of an oxygen-containing propagating gas throughthe input well towards an adjacent output well at an average input rateof oxygen sufiicient to maintain a peak tcmperaturc in the 'wave abovethe ignition point, injecting water into the underground formation fromsaid input well bore after the heat content of the wave duringpropagation is sufficient to provide a peak temperature in the wave whenit reaches the output well equivalent to the boiling point of water atthe pressure prevailing in the output well, thereafter discontinuing theinjection from the input well of any propagating gas and carrying theheat wave to the output well by further injection of water into theformation from the input well.

2. The process of claim 1 in which the oxygencontaining gas is air.

3. The process of claim 1. in which the oxygencontaining gas is amixture of air and a combustible fuel.

4. The process of claim 1 in which water in dispersiblc quantities isinjected with the oxygen-containing propagating gas.

Refereuces Cited in the file of this patent UNIT ED STATES PATENTS1,491,138 Hixon Apr. 22, 1924- 2,390,770 Barton et al. Dec. 11, 19452,584,606 Merriam et al. Feb. 5, 1952 2,642,943 Smith et al. June 23,1953

1. IN TAHE RECOVERY OF OIL FROM AN OIL-BEARING UNDERGROUND FORMATION BYTHERMAL MEANS WHEREIN A HEAT WAVE IS ESTABLISHED WITHIN THE FORMATION,THE STEPS COMPRISING HEATAING THE FORMATION AROUND THE BORE OF AN INPUTWELL TO ESTABLISH A HEATED ZONE IN THE FORMATION THEREABOUT HAVING ATEMPERATURE ABOVE ABOUT 500*F., MOVING THE SAID HEATED ZONE OUT INTOTAHE FORMATION AND COOLING THE INPUT WELL BORE BY INJECTION OF A COOLINGNON-COMBUSTING GAS THEREIN, AND THEREAFTER DISCONTINUING THE USE OF THEINPUT WELL BORE AS A HEAT SOURCE, PROPAGATING THE HEAT WAVE WITHIN THEFORMATION BY INJECTION OF AN OXYGEN-CONTAINING PROPAGATING GAS THROUGHTHE INPUT WELL TOWARDS AN ADJACENT OUTPUT WELL AT AN AVERAGE INPUT RATEOF OXYGEN SUFFICIENT TO MAINTAIN A PEAK TEMPERATURE IN THE ABOVE THEIGNITION POINT, INJECTING WATER INTO THE UNDERGROUND FORMATION FROM SAIDINPUT WELL BORE AFTER THE HEAT CONTENT OF THE WAVE DURING PROPAGATION ISSUFFICIENT TO PROVIDE A PEAK TEMPERATURE IN THE WAVE WHEN IT REACHES THEOUTPUT WELL EQUIVALENT TO THE BOILING POINT OF WATER AT THE PRESSUREPREVAILING IN THE OUTPUT WELL, THEREAFTER DISCONTINUING THE INJECTIONFROM THE INPUT WELL OF ANY PROPAGATING GAS AND CARRYING THE HEAT WAVE TOTHE OUTPUT WELL BY FURTHER INJECTION OF WATER INTO THE FORMATION FROMTHE INPOUT WELL.